A typical cryopump system is disclosed in U.S. Pat. No. 4,446,702 to J. Peterson and A. Bartlett. In that system, shown in FIG. 1, a work chamber 10 is maintained at a high vacuum by a cryopump 12. When the system is initially started, however, the work chamber 10 is brought to an intermediate vacuum pressure by a roughing pump 14. The roughing pump 14 also initially pumps down the cryopump 12 to a moderate vacuum. After the work chamber 10 and cryopump 12 have been evacuated to a moderate vacuum pressure, the roughing pump 14 is inactivated.
Additionally, in most operations it is necessary to transfer materials into and out of the high vacuum working space. Conventionally, work material is moved into the high vacuum space by first exposing it to a vacuum load lock or cross over chamber 16. Material is placed in the load lock 16 and this space is evacuated to an intermediate pressure by a roughing pump 14. In this crossover chamber, pressure is typically limited to the limitations of the roughing pump. Roughing pumps used in this system are limited to minimum pressures in the range of 400 millitorrs to minimize the effect of oil backstreaming. Above 400 millitorrs pump pressures keep gas flow in the viscous range. At lower pressures, oil vapor is released from the roughing pumps 14 and enters the work chamber 10 by molecular backstreaming. Thus, if the pressure is too low, oil vapor from the roughing pumps mix with residual gas in the crossover area. The residual gas (which typically consists of a majority of water vapor with lesser amounts of atmospheric gases and possibly oil vapor) in the crossover area is then released into the working space when work material is transferred from the crossover area into the work space. Impurities introduced in such a manner can be detrimental to high vacuum operations such as integrated circuit manufacture.
Presently, impurities are handled by the condensing arrays 18 of the cryopump 12 which maintains the high vacuum environment of the working space. The disadvantage of this method is that processing time is affected. In many cases, work space pressure is increased to a level far too high for the affected manufacturing process to continue. Work must, therefore, cease periodically during the evacuation of the contaminated crossover gas from the work chamber.
In some systems, a second cryopump coupled to the load lock 16 has been used to reduce the crossover pressure and minimize the gas pulse during the transfer of material. Such a system, however, increases the expense and the size of the over-all packaging of the system.
There exists, therefore, a need to eliminate contamination created by the roughing pump while evacuating the work environment and the cryopump to an intermediate vacuum pressure.